Biology Reference
In-Depth Information
who first drew attention to these opposing effects of two important fluid
properties of nectar. At low concentrations, energy intake is limited by the
low energy content, while at high concentrations it is limited by the high vis-
cosity of the solution. The balance between costs and benefits determines the
optimal nectar concentrations for different nectar feeders. Concentrations
predicted by modelling studies are supported to varying extents by empirical
work (see Nicolson, 2007, Chapter 7 in this volume).
Mouthpart structures and the type of fluid feeding must be considered
along with the physical properties of the solution being ingested. Two prin-
cipal mechanisms are involved in nectar uptake by insects: either the mouth-
parts use capillary action in licking or lapping nectar, or they are modified to
form long tubes for sucking nectar (Kingsolver & Daniel, 1995; Krenn et al.,
2005). Biophysical models (Kingsolver & Daniel, 1995) predict that lower
solute concentrations are necessary for efficient injestion through narrow
tubes, and this is illustrated by the relatively low concentrations of butterfly
nectars compared to those utilized by bees. Although most bees are capillary
feeders, suction feeding has evolved in the Neotropical euglossine bees,
which collect more dilute nectars than other sympatric bees (Borrell, 2004).
For bumblebees, Harder (1986) explained the depressive effect of viscosity
on ingestion rates as follows: the licking rate is constant, and provided the
tongue becomes saturated at each lick, the volume ingested will be constant.
However, when concentrations reach 35-40% or higher, the increased vis-
cosity begins to reduce the volume taken up during each immersion of the
tongue. This explains the finding that energy uptake rates are maximal at
50-65% for many bees (Roubik & Buchmann, 1984; Harder, 1986), but
lower for the euglossine bees (Borrell, 2004). Similarly, energy intake rates
are higher in the more primitive ponerine ants, which lick sugary food, than
in formicine ants, which are suction feeders (Paul & Roces, 2003).
Nectarivorous birds also lick nectar from flowers, and the nectar flows by
capillarity onto their grooved tongues. The biophysical model developed for
hummingbird feeding by Kingsolver and Daniel (1983) suggested that the
optimum nectar concentration for hummingbirds is 20-25% for small vol-
umes (involving single licks of the tongue), but higher for larger volumes.
For nectar feeders in general, regardless of the feeding mechanism used,
Heyneman (1983) predicted 22-26% as an optimum concentration for large
volumes, with the proviso that this would increase if travel costs were high
in relation to total foraging costs; (see also Harder, 1986). The temporal
scale is crucial in modelling hummingbird feeding: Gass and Roberts (1992)
demonstrated upward shifts in optimal concentration as they considered in
turn the tongue loading phase of the licking cycle, the whole licking cycle, and
